There are various applications where it is necessary to use a stack of batteries. The batteries are typically arranged in series with one another to provide a power supply of a particular rated voltage to drive an electrical load. Battery stacks can be used as an energy store in electric or hybrid electric cars. Each battery within the overall stack comprises a number of individual battery cells. The lifetime of the batteries is strongly dependent on the way in which the batteries are charged and discharged. For Lithium-ion battery cells, the remaining capacity of the battery cells is directly proportional to the open-circuit voltage. Use of the battery and over-discharge of one cell will impact the lifetime of that cell and of the total battery. As explained in U.S. Pat. No. 6,891,352 a difference of 50 mV corresponds to a 5% difference in capacity of the cell. To prolong the lifetime of the cells the difference in voltage between the cells should be as low as possible, e.g. lower then a few tens of millivolts, such as less than 25 mV. It is also important to ensure that batteries (and individual cells within batteries) perform equally well. In view of the above, it is important to monitor the voltage accurately across each cell within a battery.
It has been found that a main error in a battery monitoring control unit is the voltage reference source, which is typically only accurate to +/−2% over the full temperature range. For a maximum cell voltage of 4.2V this gives a possible range of error of +/−84 mV, which is unacceptably high.
U.S. Pat. No. 6,891,352 describes apparatus for controlling a number of batteries. A control device is provided in each battery. The control device measures the voltage across each cell using a comparator, an analog-to-digital converter and a local voltage reference. An error compensation means compensates for the error of each voltage source and, in use, the control device applies an amount of compensation to a measurement.
It is known for each battery to include an integrated control unit. Where each battery has a local control unit, it is necessary to provide communication between the local control units and a main controller. U.S. Pat. No. 6,891,352 provides a local control unit at each battery and arranges the control units in a daisy-chain configuration. The control units at each end of the chain connect to a main control unit via opto-couplers. The use of an optical connection can reduce the effects of electrical interference on data and allow the level-shifting between the voltage difference between the battery-stack and the main-controller. The output toggles within its own supply range. The input of the next device will see a voltage higher then its own supply. Arranging the controllers in a chain minimises the number of opto-couplers and connecting lines that are required.
U.S. Pat. No. 6,404,166 similarly provides a cell monitoring device at each of a plurality of battery cells and arranges the cell monitoring devices in a daisy-chain configuration. The control units at each end of the chain connect to a central battery monitoring system. Measurements are passed from one cell monitoring device to the next cell monitoring device as binary data levels. An interface between each cell monitoring device uses a level-shifter to ‘shift’ the voltage levels from the range seen by one cell monitoring device to the range seen by the next cell monitoring device. However, the use of level-shifters is undesirable as the speed of the level-shifters is dependent on the value of the resistors used and the current that will flow. When using a daisy-chain the cell-voltage of all battery-cells has to be sent to the main-controller. For example, for a stack of 80 Lithium-ion cells this gives 80×10 bit or 800 bit. Together with the communication commands, the error-detection bits and start and stop of the commands the data can come to more then 2000 bits. If the main-controller cannot accept more then 100 ms delay between 2 measurements of all cells then the required data rate is 20 kbit/s. At that moment the slopes can be not more then a few micro-seconds. The level-shifters can be made faster but then they will consume more current during the active state.
Level-shifters also have the problem that they communicate between different supplies. These supplies are not identical and will see different noise signals during use of the battery-stack (noise coupled through the supply to the signals). This will require an extra filtering at the receiving input and higher voltage swing to increase the noise margin.